Support multiline examples with DFToVW

[jackgerrits]

There was significant discussion in the original PR about how to support multiline example the the DFToVW system.

In a nutshell:

• Define a column which specifies an example id, each row that has the same id is part of the same multiline
• Most multiline formats require different example types - therefore you you must be able to define several formulae which can be selected based on the value of a column.

See the original PR for the full discussion: #2426

[etiennekintzler]

Also, since it's a whole new field I think a new class method could be created (as we did for the "from_colnames" convenience method). We then would have :

• DFtoVW: for the default constructor
• DFtoVW.from_colnames: for the easy interface
• DFtoVW.multiline: for the multiline cases

A proper class method for multiline would prevent the default constructor to be clogged with more parameters and would bring better separation of concerns.

[jackgerrits]

Yeah I like the idea of keeping it separate.